A number of methods for packing sand molds in foundries are already known, including designs in which a pressure impulse for packing the foundry sand is produced by an exothermic reaction of a gas. Because the fuel gas must be supplied to the combustion chamber in predetermined quantities or mixture proportions in order to obtain the same degree of compaction for all molds, the prior art includes two different quantity measurement principles.
According to one of these principles, the gas is received from a gas source at a constant pressure for a definite interval of time through a defined flow resistance so that a defined gas volume at normal pressure, or a defined quantity by weight, is received. In accordance with the other principle, the volume is directly measured by means of a gas meter. In either case, the measurements are rather inexact so that variations in the fuel mixture occurred, resulting in non-reproducible degrees of compaction of the molds. In the first case, obtaining constant pressure within narrow limits is possible only with large and expensive control and monitoring equipment. In the second case, the pressure is not at all controlled and it is just assumed that the pressure is constant, an assumption which is generally not correct. However, the major disadvantage of these known methods is the fact that the operating procedures are too slow and, consequently, can not be used in fixed-cycle molding machinery.